Method for Microfabrication of a Capacitive Micromachined Ultrasonic Transducer Comprising a Diamond Membrane and a Transducer Thereof

ABSTRACT

This invention relates generally to capacitive micromachined ultrasonic transducers (CMUTs), particularly to those comprising diamond or diamond like carbon membranes and a method of microfabrication of such CMUTs, wherein the membrane of diamond or diamond like carbon is attached to the substrate by plasma-activated direct bonding of an interlayer of high temperature oxide (HTO).

FIELD OF THE INVENTION

This invention relates generally to capacitive micromachined ultrasonictransducers (CMUTs), particularly to those comprising diamond or diamondlike carbon (will be referred to as diamond) membranes and a method ofmicrofabrication of such CMUTs.

BACKGROUND OF THE INVENTION

Capacitive micromachined ultrasonic transducers (CMUTs) areelectromechanical energy conversion devices used to transmit and receiveultrasound. CMUTs used in immersion are generally composed ofvacuum-sealed cavities formed by a membrane material. The vacuum-sealedcavities are conventionally realized by two techniques. First one is thesacrificial release process where sacrificial material deposited beforethe membrane material is etched through the etch holes, and etch holesare filled by deposition under low pressure to form the cavity. CMUTsfabricated by sacrificial release process mostly feature Si₃N₄membranes. The limitations of sacrificial release process to achievevery large membranes without breaking or very small membranes with highfill factor adversely affect the precise engineering of the transducerphysical parameters. The Si₃N₄ membranes are also hard to presentwell-controlled deflection profiles due to the process-dependentresidual stress in the membrane material. The second one is direct waferbonding method where two wafers (one having cavity patterning and theother having the membrane material) are bonded under elevatedtemperatures under vacuum.

Among these microfabrication methods, direct wafer bonding technology ismore economical offering better process control, higher yield, and morenovelties in CMUT designs than the sacrificial release process. Directwafer bonding technology enabled development of single crystal siliconmembrane CMUTs rather than silicon nitride membrane ones. As themembrane and the substrate material are both silicon, direct waferbonding at high temperatures (1100° C.) is achieved without introducingany residual stress in the membrane. Furthermore, IC compatible directwafer bonding at lower temperatures (400° C.) can be also utilized. Thistechnology has significantly reduced the complexity and the time of theprocessing of CMUTs additionally offering superior process control, highyield, and improved uniformity compared to the already maturesacrificial release process. Best part of wafer bonding technology is topresent a well-known silicon crystal material as a membrane, and toachieve vacuum sealed cavities without the need to open etch holes onthe membrane, both of which directly translate into reliable operationin immersion. Recently, successful flip-chip bonding of IC andwafer-bonded 2-D CMUT arrays incorporating through wafer trench-isolatedinterconnects has been demonstrated. Therefore, recent developmentsenable the specifications of the CMUT design be comfortably satisfiedfacilitating the realization of industrial grade CMUT products usingdirect wafer bonding technology.

Energy conversion efficiency of CMUTs has been of primary importance forultrasound applications, and improvement of this efficiency has beenextensively studied for ultrasound transducers. Conventionally, the CMUTis biased at a voltage below the collapse voltage, and an AC signal isapplied to generate ultrasound. The efficiency of the transducer isdrastically improved as the bias voltage approaches the close vicinityof the collapse voltage. However, this high efficiency comes with a riskof membrane collapse onto the substrate. Additionally, the AC amplitudeis limited to a small excitation voltage around a large bias voltage toprevent membrane collapse during operation. Therefore, the maximumoutput pressure of a CMUT is inherently limited by the requirements ofthe conventional operation.

For potential applications such as high intensity focused ultrasound(HIFU) in medical therapeutics, larger output pressures are essential.To offer unprecedented acoustic output pressure in transmit without theaforementioned limitations, novel CMUT operation modes of collapse andcollapse-snapback are introduced. Both operation modes require themembrane to contact the substrate surface, which poses a problem on thedurability of the membrane in terms of structural integrity andtribological property. Large membrane deflection at collapse increasesthe stress within the membrane, and change of stress at ultrasoundfrequencies causes reduced lifetime and compromised reliability in thesehigh output pressure operation modes. Ultrasound applications requirethe transducer surface to be in contact with the acoustic medium.Because the surface is subject to environmental conditions as well asexternal pressures, the durability of the membrane defined by hardnessis also a major criteria for CMUT performance. Because of electrostaticforces in addition to the atmospheric pressure due to the vacuum sealedcavities, Young's modulus of the membrane plays an important role in themembrane deflection profiles as well.

Collapse-snapback mode requires the collision of the contacting surfacesevery cycle, and heat released needs to be dissipated quickly tomaintain stable operation. Based on the additional requirements of thesemodes to reach high output transmit pressure at a sustainable transduceroperation, diamond is proposed as the ultimate solution to be used asthe membrane material. Mechanical (high Young's modulus, extremehardness), thermal (large thermal conductivity, low thermal expansioncoefficient), and electrical properties (insulator, large electricalbreakdown field) of diamond are all in favor of its use in themicrofabrication of CMUTs. Chemical inertness, biocompatibility andsurface modification are further benefits of diamond for CMUTs to beutilized in corrosive environment and biological samples, respectively.For example, hydrophilic O₂-terminated diamond surface, achieved byoxygen plasma or piranha wet processing, will withstand against thedetrimental cavitation shock of bubbles in immersion. Because no wetchemical etchant of diamond exists, its use is best suited for extremeand harsh environments. Compared to all potential membrane materials aswell as current membrane materials of Si₃N₄ and silicon, diamonddistinguishes itself based on high Young's modulus and exceptionalhardness (see Table 1 for material properties of Si₃N₄, Si, anddiamond).

TABLE 1 Material Property Silicon Nitride Silicon Diamond Young'sModulus (GPa) 320 160 1200 Hardness (kg/mm²) 1580 1000 10000 ThermalConductivity (W/mK) 30 151 2000 Thermal Expansion (10⁻⁶/K) 3.3 2.5 1.1

Diamond is a perfect membrane material candidate based on its materialproperties. However, unmature single crystal diamond (SCD) depositiontechnologies prevented diamond membranes integration into CMUTs. Thinfilm SCD coated wafers are not commercially available for batch MEMSprocesses. Surface roughness of SCD is also high to be utilized for CMUTmicrofabrication based on direct wafer bonding technology.

Recently, with improvements in diamond material growth and technology,ultrananocrystalline diamond (UNCD) as a thin film was made commerciallyavailable.

UNCD shares a large portion of the benefits of the SCD with compromisedfeatures such as reduced resistivity due to graphitic forms enclosingpolycrystalline diamond (SCD: insulator, UNCD: highly resistive). Aremarkable feature of UNCD as a membrane material is its deposition as athin film over a wafer surface with very low residual stress (i.e. <50MPa). UNCD, featuring smaller grain size and surface roughness has beenrecently explored for microelectromechanical systems (MEMS) applicationssuch as RF MEMS resonators and hybrid piezoelectric/UNCD cantilevers.However, there are no studies of CMUTs with diamond membranes.

In the documents U.S. Pat. No. 7,846,102B2 and U.S. Pat. No. 7,745,248B2disclosing various improvements regarding CMUTs, it has been merelymentioned that diamond can be used in the membrane material amongstother materials such as silicon, silicon nitride or silicon carbide.

In the document U.S. Pat. No. 7,530,952B2, a CMUT incorporating directwafer bonding between the membrane and the substrate is disclosed. Ithas also been mention in said document that the membrane material can beof diamond amongst other materials such as silicon, silicon nitride orsapphire.

The inventions disclosed in the above mentioned documents, U.S. Pat. No.7,846,102B2, U.S. Pat. No. 7,745,248B2 and U.S. Pat. No. 7,530,952B2,are not concerned with providing a method to use diamond in the membraneand thus, neither the characteristics of the diamond material to be usednor the means for such use of diamond are not established.

An inconvenience arises in the use of diamond in a membrane to be joinedto the substrate by direct wafer bonding, due to the surface propertiesof diamond layers grown on a substrate as is required for direct waferbonding. The high surface roughness of such a diamond layer and the lowchemical affinity between diamond and silicon dioxide hinders theestablishment of the desired direct wafer bond. Moreover, applyingconventional polishing methods on a diamond layer does not improve thedirect wafer bonding abilities of the diamond layer.

SUMMARY OF THE INVENTION

The object of the invention is to achieve microfabrication of a CMUTemploying a membrane having ultrananocrystalline diamond (UNCD) ornanocrystalline diamond (NCD). A membrane made of diamond advances thestate of the art CMUT features due to several advantageous diamondmaterial properties such as high Young's modulus, high hardness, highheat conductivity and low thermal expansion.

Another object of the invention is to achieve microfabrication of adiamond-based CMUT by plasma-activated direct wafer bonding ofcavity-defined thermally oxidized silicon wafer and diamond coatedsilicon wafer having a thin high temperature oxide interlayer on top.

The method for microfabrication of a diamond-based CMUT having at leastone

CMUT cell, basically includes the steps

1. preparation of the base substrate by

-   -   a. preparing a first electrode layer on a substrate, preferably        by doping of the surface of said substrate and forming a        conductive top surface suitable for ohmic contact with a metal;    -   b. and forming cavity walls on said doped substrate surface by        growing, patterning via lithography mask, and then etching        silicon dioxide;

2. preparation of the membrane substrate by

-   -   a. preparing or obtaining of diamond layer coated on a        substrate;    -   b. depositing electrically-insulating high temperature oxide on        diamond as an intermediate layer.    -   c. and thinning the intermediate layer to a final thickness via        chemical mechanical polishing (CMP) to achieve reduced surface        roughness of the top interlayer surface for direct wafer        bonding;

3. bonding of the base and the membrane substrates by

-   -   a. cleaning and plasma activation of both surfaces of the base        and the membrane substrates;    -   b. contacting of both surfaces in high vacuum conditions (10⁻⁴        mbar) such that the intermediate layer is facing cavities;    -   c. annealing the pre-bonded substrates under 10 kN contacting        force at 550° C. for several hours;

4. removing the substrate of the diamond layer;

5. and forming at least one second electrode on the membrane and atleast one first electrode connection;

wherein the intermediate layer of step 2.b. is of a material with highchemical affinity towards the cavity wall material and by polishing thesurface roughness of said intermediate layer is decreased afterdeposition, to below 0.5 nm thus providing a surface suitable for directwafer bonding. The resulting thickness of said intermediate layer afterbeing polished is determined such that the behavior of the membrane isdetermined by the diamond layer.

Preferably, for the diamond layer, diamond in the form of ananocrystalline diamond (NCD) or an ultrananocrystalline diamond (UNCD)layer with a low residual stress below 50 MPa is obtained on a siliconor silicon dioxide wafer and then a high temperature oxide (HTO) to formthe intermediate layer is applied on the diamond layer. The HTO is thenchemically-mechanically polished (CMP) to obtain the desired surfaceroughness and thickness. Shaping of various layers can be performedusing known masking, etching etc. methods.

Thus the CMUT according to the invention consists of a base wafer, aconductive layer to act as a first electrode on one surface of said basewafer, at least one cavity on said first electrode, an intermediatelayer adjacent to said at least one cavity, a diamond membrane, at leastone second electrode on the membrane and at least one first electrodeconnection.

The diamond layer may be undoped or doped. If the diamond used for themembrane is of a conductive form, then the membrane itself acts as asecond electrode. Then, the intermediate layer serves also as anelectrical insulator preventing the circuit from shorting through thefirst electrode.

BRIEF DESCRIPTION OF THE FIGURES

The objectives and advantages of the present invention will beunderstood by reading the following detailed description in conjuctionwith the drawing, in which:

FIG. 1 is a planar cross-section view of the CMUT cell according to theinvention.

FIG. 2( a) shows a top view of a single CMUT.

FIG. 2( b) shows a magnified view of a CMUT cell and its neighboringcells.

FIG. 3( a) shows a top view of a 16-element 1-D CMUT array.

FIG. 3( b) shows a magnified view of a portion of 5 neighboring 1-D CMUTarray elements.

FIG. 3( c) shows a magnified view of a CMUT cell and its neighboringcells.

FIG. 4( a) shows an n-type silicon substrate with <100> crystalorientation.

FIG. 4( b) shows an n-type silicon substrate with phosphorous doped(n⁺-type) conductive surface layer.

FIG. 4( c) shows thermally oxidized silicon dioxide layer on top ofn⁺-type surface layer.

FIG. 4( d) shows spin-coated photoresist layer on top of thermal silicondioxide layer of FIG. 4( c).

FIG. 4( e) shows patterned photoresist layer of FIG. 4( d) vialithography mask CAVITY.

FIG. 4( f) shows patterned thermal silicon dioxide layer (protected viapatterned photoresist) via reactive ion etching of silicon dioxide(RIE-SiO₂) using CHF₃/CF₄ gas chemistry.

FIG. 4( g) shows cavity walls of thermal silicon dioxide formed overdoped silicon substrate (photoresist removed via oxygen plasma).

FIG. 5( a) shows diamond deposited on silicon wafer.

FIG. 5( b) shows high temperature oxide (SiO₂) deposited on siliconsubstrate in a low pressure chemical vapor deposition (LPCVD) furnaceusing dichlorosilane (SiH₂Cl₂) and nitrous oxide (N₂O) gas chemistry.

FIG. 5( c) shows chemically mechanically polished (CMP) high temperatureoxide layer (thinned) of FIG. 5( b).

FIG. 6( a) shows plasma-activated direct wafer bonded pair of topsurfaces of substrates in FIG. 4( g) and FIG. 5( c).

FIG. 6( b) shows thinned substrate (mechanically supporting the diamondlayer) of FIG. 6( a) via grinding.

FIG. 6( c) shows plasma enhanced chemical vapor deposition (PECVD) ofsilicon dioxide on the bottom of the substrate having cavity walls.

FIG. 6( d) shows diamond membranes over cavity via wet chemically etchedthinned substrate of FIG. 6( b).

FIG. 7( a) shows plasma enhanced chemical vapor deposition (PECVD) ofsilicon dioxide on top of the diamond membrane of FIG. 6( d).

FIG. 7( b) shows spray-coated photoresist layer on top of PECVD silicondioxide of FIG. 7( a).

FIG. 7( c) shows patterned photoresist layer of FIG. 7( b) vialithography mask CONTACT.

FIG. 7( d) shows patterned PECVD silicon dioxide layer (protected viapatterned photoresist) via reactive ion etching of silicon dioxide(RIE-SiO₂) using CHF₃/CF₄ gas chemistry.

FIG. 7( e) shows patterned diamond layer (protected via patterned PECVDsilicon dioxide layer) via reactive ion etching of diamond (RIE-C) usinginductively coupled O₂ plasma.

FIG. 7( f) shows opening of ground contact area on doped substrate andtop electrode contact area on diamond membrane via reactive ion etchingof silicon dioxide (RIE-SiO₂) using CHF₃/CF₄ gas chemistry.

FIG. 8( a) shows aluminium deposited on top surface of FIG. 7( f).

FIG. 8( b) shows spray-coated photoresist layer on top surface of FIG.8( a).

FIG. 8( c) shows patterned photoresist layer of FIG. 8( b) vialithography mask METAL.

FIG. 8( d) shows planar cross-sectional view of the final device havingpatterned aluminium layer (protected via patterned photoresist) via wetchemical etching, and removal of the patterned photoresist via oxygenplasma.

FIG. 9( a) is a graph showing the experimental and the theoreticaldeflection profiles of a CMUT with a nanocrystalline diamond membraneaccording to the invention.

FIG. 9( b) is a graph showing the experimental and the theoreticaldeflection profiles of a CMUT with a ultrananocrystalline diamondmembrane according to the invention.

FIG. 10( a) is a graph of capacitance and resistance versus frequencyfor a CMUT according to the invention.

FIG. 10( b) is a graph showing the experimental and the theoreticaldeflection profiles versus bias voltage of a CMUT according to theinvention.

FIG. 11 is a graph of hydrophone readings with respect to time relatingto an airborne CMUT according to the invention.

FIG. 12( a) is a photo of aligned diamond-based CMUT and needlehydrophone in immersion. 2-D scan area in x and y coordinates are shownvisually. Origin corresponds to the center of the CMUT.

FIG. 12( b) is a graph of measurement results of the normalizedpeak-to-peak pressure (in dB) for 2-D scan area. Theoreticallycalculated lines separating the main lobe and the side lobes are alsoshown with dotted lines on top of the measurement data.

FIG. 13( a) is a graph of experimental and theoretical results of thenormalized peak-to-peak pressure on the normal of the CMUT surface.

FIG. 13( b) is a graph of experimental acoustic output pressure alongthe x-axis parallel to the CMUT surface at y=15 mm (Fresnel distance(S=1)), y=30 mm (S=2), and y=8.2 mm (S=0.5).

FIG. 13( c) is a graph of spectrum of the diamond-based CMUT withpeak-to-peak AC amplitudes of 9 V, 36 V, and 54 V.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Although the following detailed description contains many specifics forthe purposes of illustration, anyone of ordinary skill in the art willreadily appreciate that many variations and alterations to the followingexemplary details are within the scope of the invention.

Accordingly, the following preferred embodiment of the invention is setforth without any loss of generality to, and without imposinglimitations upon, the claimed invention.

A CMUT cell produced according to the present invention is shown inFIG. 1. Said CMUT cell consists of a silicon substrate base wafer 100, asubstrate electrode layer 102 produced on one surface of said substratebase wafer 100 by doping said surface, cavity walls 114 of a thermaloxide on said substrate electrode layer 102 for defining at least onecavity, a diamond layer 142 to provide membrane functions, anintermediate layer 144 of HTO deposited on one side of said diamondlayer 142 and providing a surface for direct wafer bonding between saidcavity walls 114 and said diamond layer 142, a membrane electrode 156 onthe other side of the diamond layer 142, and substrate electrodeconnections 158 (not shown in FIG. 1) formed on the substrate electrodelayer 102. The diamond layer 142 may be undoped or doped. When thediamond layer 142 is doped, the diamond layer 142 itself acts as anelectrode and the membrane electrode 156 is rendered obsolete thus it iseither used merely as an electrode connection or is not fabricated atall.

The substrate electrode layer 102 can be n-doped or p-doped, usingdopants such as phosphorus or boron respectively.

NCD consists of nanocrytalline diamond each of which having a grain sizeof 10 nm, whereas UNCD consists of ultrananocrystalline diamond each ofwhich having a grain size of 3 to 5 nm.

During CMP, a reduction of 0.3 μm is sufficient to decrease the surfaceroughness of the intermediate layer 124 of HTO in FIG. 5( c) to a valuebelow 0.5 nm. Thus the originally deposited HTO 122 in FIG. 5( b) has athickness 0.3 μm more than the desired thickness.

The membrane electrode 156 and the substrate electrode connections 158in FIG. 8( d) can be of any combination of metals such as aluminum,titanium, platinum or gold.

The dimensions of the CMUT cell is determined according to theoperational characteristics of the ultrasound transducer such ascollapse voltage and center frequency.

The ratio of the thickness of the intermediate layer 144 to that of thediamond layer 142 cannot exceed 5 such that the behavior of the membraneis determined by the diamond layer 142. Said ratio is preferably 0.25.

Some dimensions have been marked on the CMUT cell depicted in FIG. 1where; t_(s) is the substrate base wafer 100 thickness including thesubstrate electrode layer 102, t_(g) is the thickness of the cavitywalls 114 and thus the cavities, t_(h) is the thickness of theintermediate layer 144, t_(m) is the thickness of the diamond layer 142,t_(e) is the thickness of the membrane electrode 156, s is the supportlength of a CMUT cell excluding the substrate electrode connections 158,r_(m) is the radius of a cavity and thus the active region of themembrane, and r_(e) is the radius of the membrane electrode 156.

For a CMUT cell according to the invention the above defined dimensionsare

-   -   t_(s): approximately 500 μm, insignificant in scope of the        invention    -   t_(g): 0.1-5 μm, as is known from the prior art    -   t_(h): 0.1-1 μm    -   t_(m): 0.3-10 μm    -   t_(e): 0.2-2 μm, as is known from the prior art    -   s: 1-100 μm, as is known from the prior art    -   r_(m): 5-1000 μm, as is known from the prior art    -   r_(e): 5-1000 μm, as is known from the prior art

A CMUT producing ultrasound vibrations of a frequency of 1.74 MHz in airunder a DC voltage of 100 V, fabricated according to the invention wasused for testing purposes. The dimensions of each CMUT cell accordinglyare

-   -   t_(s): 525 μm    -   t_(g): 1.57 μm    -   t_(h): 0.23 μm    -   t_(m): 1.0 μm    -   t_(e): 0.4 μm    -   s: 3 μm    -   r_(m): 60 μm    -   r_(e): 30 μm

A single CMUT design in FIG. 2 and 1-D CMUT array design in FIG. 3 arerealized successfully using the present invention.

For the above mentioned CMUT, the method for microfabrication of a CMUTcomprising a diamond membrane was performed through the steps:

1. preparation of the substrate by

-   -   a. preparing an n-type silicon wafer as a substrate base wafer        100 of 500 μm to act as a base for the CMUT as depicted in FIG.        4( a);    -   b. doping the surface of the substrate base wafer 100 with        phosphorous, by processing in a doping furnace with POCl₃ and O₂        at 1050° C., to obtain a conductive substrate electrode layer        102, with a conductivity less than 1 Ω/square, as depicted in        FIG. 4( b);    -   c. and forming cavities by        -   i. depositing, at 1000° C., a thermal oxide of silicon            dioxide on the substrate electrode layer 102 as depicted in            FIG. 4( c);        -   ii. coating said thermal oxide layer with a photoresist 106            as depicted in FIG. 4( d);        -   iii. etching said photoresist 104 with UV radiation 112            while preserving the photoresist regions corresponding to            the desired cavity wall positions using a mask 110 as            depicted in FIG. 4( e);        -   iv. forming the cavity walls 114 by reactive ion etching            (RIE) 116 of regions of said thermal oxide layer not covered            by the photoresist 108, using CHF₃ and CF₄ as depicted in            FIG. 4( f);        -   v. and removing the leftover photoresist 108, using O₂            plasma, as depicted in FIG. 4( g);

2. preparation of the membrane by

-   -   a. preparing or obtaining of a silicon wafer as a membrane base        wafer 118, coated with a diamond layer 120 of UNCD or NCD whose        residual stress is lower than 50 MPa, as depicted in FIG. 5( a);    -   b. forming a HTO intermediate layer 122 of silicon dioxide on        said diamond layer 120, by low pressure chemical vapor        deposition at 850° C. using SiH₂Cl₂ and N₂O as depicted in FIG.        5( b);    -   c. and adjusting the surface roughness and thickness of said        intermediate layer 124, by chemical mechanical polishing such        that the surface roughness is in a small vicinity of 0.3 nm, as        depicted in FIG. 5( c);

3. assembling the membrane on the substrate, after activation of therespective surfaces with N₂ plasma and at 550° C., under a vacuum of10⁻⁴ mbar and a force of 10 kN for 7 hours, such that the intermediatelayer 124 is facing cavities, by direct wafer bonding between theintermediate layer 124 and the cavity walls 114 as depicted in FIG. 6(a);

4. removing the membrane base wafer 118 by

-   -   a. optionally, decreasing the thickness of the membrane base        wafer 118 to 100 μm by grinding, in order to decrease etching        time, as depicted in FIG. 6( b);    -   b. coating the substrate base wafer 100 with a protective layer        of SiO₂ 128, by plasma enhanced chemical vapor deposition        (PECVD), to provide protection of the substrate base wafer 100        as depicted in FIG. 6( c);    -   c. removing the membrane base wafer 126 by etching with        tetramethylammonium hydroxide as depicted in FIG. 6( d);

5. and forming the membrane electrode 156 and the substrate electrodeconnections 158 by

-   -   a. coating the diamond layer 120 with a protective layer of SiO₂        130, by PECVD, to provide protection of the diamond layer 120 as        depicted in FIG. 7( a);    -   b. coating the last mentioned protective layer 130 with a        photoresist 132 as depicted in FIG. 7( b);    -   c. etching said photoresist 130 with UV radiation 112 while        preserving the photoresist regions 134 corresponding to the        desired membrane shape and substrate electrode connection        positions 158 using a mask 136 as depicted in FIG. 7( c);    -   d. reactive ion etching 116 of regions of the last mentioned        protective layer 138 not covered by the photoresist 134 as        depicted in FIG. 7( d);    -   e. reactive ion etching 140 of diamond 142 not covered by the        last mentioned protective layer 138, and removing the leftover        photoresist 134 via inductively coupled oxygen plasma as        depicted in FIG. 7( e);    -   f. reactive ion etching 116 of high temperature oxide layer 144,        thermal oxide cavity wall 146, and protective layer of SiO₂ 130,        while diamond 142 acts as etch stop as depicted in FIG. 7( f);    -   g. forming a metal coating of aluminum 148 at the top of the        CMUT by sputtering as depicted in FIG. 8( a);    -   h. coating said metal coating 148 with a photoresist 150 as        depicted in FIG. 8( b);    -   i. etching said photoresist 150 with UV radiation 112 while        preserving the photoresist regions 154 corresponding to the        desired membrane electrode 156 shapes and substrate electrode        connection 158 positions, using a mask 152 as depicted in FIG.        8( c);    -   j. wet chemical etching of regions of the metal coating not        covered by the photoresist 154 as depicted in FIG. 8( d), and        removing the leftover photoresist 154 as depicted in FIG. 8( d).

During the deposition of HTO in step 2.b., SiH₂Cl₂ and N₂O are employed.However, the N₂O gas, being a strong oxidizer, can damage the diamondlayer 120. Therefore this process is performed with a specific flow rateratio of SiH₂Cl₂ to N₂O equal to 1:2, leaving no excess N₂O, whereas theconventional ratio is 1:5.

Removing of a membrane base wafer 118 of 500 μm solely by etching withtetramethylammonium hydroxide takes ten to twelve hours. Therefore theoptional step 4.a. is incorporated thus decreasing the etching time toapproximately two hours.

In an embodiment of the invention, the membrane electrode 156 consistsof a titanium layer on the membrane to provide stiction, a platinumlayer on said titanium layer to act as a diffusion barrier and a goldlayer on said titanium layer.

The arrays of CMUT cells according to the invention can be of circular,polygonal or any other shape, and be arranged in various patterns byusing masks of relevant shapes. Generally, the most efficient CMUTdesign, in terms of cells per area, would consist of regular hexagonalcells. Such an array with circular cells is depicted in FIG. 3.

The deflection profiles of two CMUTs fabricated according to theinvention with membranes of NCD and UNCD are depicted in FIG. 9( a) andFIG. 9( b), respectively. The solid lines mark the measured values whilethe dashed ones mark the theoretical values which were obtained usingfinite element analysis method.

A CMUT having an inner radius of 2586 μm containing 1500 CMUT cells of acircular shape and a diameter of 120 μm was tested in air using ahydrophone. Capacitance and resistance of said CMUT was measured againsta range of frequency, and the results are shown in FIG. 10( a).Deflection versus bias voltage was measured using white lightinterferometer, and the results are depicted in FIG. 10( b). The solidlines mark the measured values while the dotted ones mark thetheoretical values which were obtained using finite element analysismethod. Moreover, a DC bias voltage of 100 V and a sinusoidal AC voltageof 1.74 MHz with a peak-to-peak voltage 35 V were applied on said CMUTfor 5 cycle burst. The hydrophone was aligned with the central normal ofthe CMUT at a 1.9 mm from the surface of the CMUT. The hydrophonereadings are with respect to time are seen in FIG. 11.

A CMUT having an inner radius of 2586 μm containing 2708 CMUT cells of acircular shape and a diameter of 88 μm was tested in sunflower oil usinga hydrophone as depicted in FIG. 12( a). A DC bias voltage of 100 V anda sinusoidal AC voltage of 3.5 MHz with a peak-to-peak voltage of 36 Vwere applied on said CMUT for 10 periods. A two dimensional ultrasoundscan was performed, and the results obtained are shown in FIG. 12( b).Moreover, normalized peak-to-peak pressure along the central normal ofsaid CMUT was measured, and the results are depicted in FIG. 13( a). Thesolid lines mark the measured values while the dotted ones mark thetheoretical values which were obtained using finite element analysismethod. Also, using the two dimensional scan data and the calibrationvalues of the hydrophone, the acoustical output pressure of the CMUTalong lines parallel to the CMUT was obtained as shown in FIG. 13( b).The data for lines at distances of 15 mm (Fresnel distance, S=1), 30 mm(S=2) and 8.2 mm (S=0.5) are depicted in FIG. 13( b). A symmetricaldouble peak at S=0.5 and a single peak with reduced magnitude at S=2were found as expected by theory. The CMUT was further tested with ahydrophone placed along the central normal at a distance of 54.1 mm fromthe surface of the CMUT. A DC bias voltage of 100 V and sinusoidal ACvoltages were applied on said CMUT for 30 periods. There different ACvoltages of peak-to-peak 9, 36 and 54 V, each being varied from 1 MHz to8 MHz with steps 100 kHz, were used. The output magnitude of the CMUTversus frequency graph for said three voltage values obtained isdepicted in FIG. 13( c).

Various embodiments and applications employing the principles of thepresent invention can be implemented. Therefore the scope of theinvention is not limited to the examples above but determined by thefollowing claims.

1. A method for microfabrication of a capacitive micromachinedultrasonic transducer (CMUT) containing at least one CMUT cellcomprising the steps preparing a substrate by preparing a firstelectrode layer on a substrate, preferably by doping of the surface ofsaid substrate; forming cavity walls on said first electrode bydepositing and then etching a cavity wall material; preparing a membraneby preparing or obtaining of diamond layer coated on a substrate;forming and polishing an intermediate layer on the diamond layer;assembling the membrane on the substrate such that the intermediatelayer is facing cavities, by plasma-activated direct wafer bondingbetween the intermediate layer and the cavity walls; removing thesubstrate of the diamond layer; and forming at least one first electrodeconnection; wherein said intermediate layer is of a material with highchemical affinity towards the cavity wall material, the surfaceroughness of said intermediate layer is decreased by polishing to below0.5 nm, and the thickness of said intermediate layer after polishing isdetermined such that the behavior of the membrane is determined by thediamond layer.
 2. The method of claim 1, wherein the cavity wallmaterial is a thermal oxide of silicon oxide and the intermediate layeris of a high temperature oxide of silicon dioxide.
 3. The method ofclaim 2, wherein the ratio of the thickness of the polished hightemperature oxide to that of the diamond layer is less than 5,preferably equal to 0.25.
 4. The method of claim 2, wherein the hightemperature oxide is deposited using SiH₂Cl₂ and N₂O and the specificflow rate ratio of SiH₂Cl₂ to N₂O is equal to 1:2.
 5. The method ofclaim 2, wherein the thickness of the high temperature oxide beforepolishing is at least 0.3 μm more than that of the polished hightemperature oxide.
 6. The method of claim 1, wherein the membrane isassembled to the substrate after surface activation with N₂ plasma andat 550° C., under a vacuum of 10⁻⁴ mbar.
 7. The method of claim 1,wherein the diamond layer is of nanocrystalline diamond.
 8. The methodof claim 7, wherein the diamond layer is on a substrate of silicon. 9.The method of claim 1, wherein the diamond layer is ofultrananocrystalline diamond.
 10. The method of claim 9, wherein thediamond layer is on a substrate of silicon dioxide.
 11. The method ofany of claims 7 to 10, wherein the diamond layer has a residual stressless than 50 MPa.
 12. The method of claim 1, wherein the first electrodeconnections are formed in at least one recess in the diamond layer. 13.The method of claim 12, wherein the recesses in the diamond layer areobtained by reactive ion etching of the diamond layer while regions ofthe diamond layer not to be etched are covered with a protective layerof silicon dioxide.
 14. The method of claim 1, wherein the diamond layeris undoped and at least one second electrode is also formed on themembrane during the last step.
 15. The method of claim 1, wherein thediamond layer is doped.
 16. The method of claim 15, wherein at least onesecond electrode to act as an electrode connection is also formed on themembrane during the last step.
 17. The method of any of claim 14 or 16,wherein the second electrode is of a combination of aluminum, platinum,titanium and gold.
 18. The method of claim 17, wherein the secondelectrode is of aluminum sputtered onto the membrane.
 19. The method ofclaim 17, wherein the second electrode consists of a titanium layer onthe membrane, a platinum layer on said titanium layer and a gold layeron said titanium layer.
 20. A CMUT consisting of at least one CMUT cell;each comprising of a silicon substrate base wafer; a substrate electrodelayer produced on one surface of said substrate base wafer by dopingsaid surface; cavity walls on said substrate electrode layer fordefining at least one cavity; a diamond layer to provide membranefunctions; an intermediate layer deposited on one side of said diamondlayer and providing a surface for direct wafer bonding between saidcavity walls and said diamond layer; and substrate electrode connectionsformed on the substrate electrode layer.
 21. The CMUT of claim 20,wherein the cavity wall is of a thermal oxide of silicon oxide and theintermediate layer is of a high temperature oxide of silicon dioxide.22. The CMUT of claim 21, wherein the ratio of the thickness of theintermediate layer to that of the diamond layer is less than 5,preferably equal to 0.25.
 23. The method of claim 20, wherein thediamond layer is of nanocrystalline diamond or ultrananocrystallinediamond.
 24. The CMUT of claim 20, wherein the substrate electrodeconnections are in recesses in the diamond layer.
 25. The CMUT of claim20, wherein the diamond layer is undoped and there is at least onemembrane electrode on the membrane.
 26. The CMUT of claim 20, whereinthe diamond layer is doped.
 27. The CMUT of claim 26, wherein there isat least one membrane electrode to act as an electrode connection on themembrane.
 28. The CMUT of any of claim 25 or 27, wherein the membraneelectrode is of a combination of aluminum, platinum, titanium and gold.29. The CMUT of claim 28, wherein the membrane electrode is of aluminum.30. The CMUT of claim 28, wherein the membrane electrode consists of atitanium layer on the membrane, a platinum layer on said titanium layerand a gold layer on said titanium layer.